A variety of kinds of power transmissions have been employed for the vehicles, in which the clutch or the transmission that transmits the power of the engine to the wheels is automatically operated for easy driving in order to facilitate the driving of a vehicle and to reduce fatigue to the driver. A representative example may be an automatic transmission comprising a torque converter which is a fluid transmission device and a planetary gear mechanism, as is widely used as the power transmission device for the so-called automatic transmission vehicles (AT vehicles).
One of the power transmission devices for vehicles designed for achieving easy driving is a power transmission device which employs the transmission of the type of parallel shaft gear mechanism similar to that of the so-called manual transmission vehicle (MT vehicle) in combination with an automatic clutch to eliminate the need of operating the clutch at the time when the driver changes the gear by using the speed-change lever. A power transmission device has also been proposed in which the gear is automatically changed over depending upon the traveling state of the vehicle by using an electronic control unit and an actuator for operating the transmission instead of operating the speed-change lever by the driver. In recent years, a power transmission device has also been placed in practical use by interposing a fluid coupling which is a fluid transmission device simpler than the torque converter between the engine and the transmission of the type of parallel shaft gear mechanism. With the fluid coupling being interposed, the vehicle starts smoothly moving owing to slipping between the pump and the turbine of the fluid coupling and, besides, fluctuation in the engine torque is absorbed during idling contributing to reducing vibration and noise.
The AT vehicles are, usually, equipped with a parking lock device. When the vehicle comes into a halt, the driver moves the speed-change lever to the parking position, whereby the drive wheels of the vehicle are locked and the vehicle is reliably maintained in the halted state. The parking lock device is arranged at a rear portion of the automatic transmission, and works to lock the output shaft of the automatic transmission and the drive wheels by fixing a parking gear to the output shaft of the automatic transmission coupled to the drive wheels or to a rotary shaft coupled to the output shaft and by bringing a parking pawl attached to the housing of the automatic transmission into mesh with the parking gear. The above parking lock device has been disclosed in JP-A-2001-341619 as one of embodiments.
The parking lock device and the operation mechanism for bringing the parking pawl into mesh taught in the above publication will now be described with reference to FIGS. 9(a)-9(c). FIG. 9(a) is a perspective view illustrating the whole constitution, FIG. 9(b) illustrates the constitution of a wedge and the like for moving the parking pawl, and FIG. 9(c) is a view illustrating the wedge after assembled in cross section.
A squarely and continuously indented parking gear 102 is fixed to an output shaft 101 of a transmission. A parking pawl 103 having a pawl portion 103A is fixed to a parking pawl shaft 103B which is supported by bearings of the housing of the transmission, the parking pawl 103 being pushed downward by a spring (torsion coil spring) that is not shown. Under the parking pawl 103, a wedge shaft 104 is arranged extending in a direction at right angles thereto. Referring to the right upper drawing, the parking pawl 103 is pushed down by the spring and is in contact with the wedge shaft 104.
Referring to the right lower drawing, the wedge shaft 104 includes a large-diameter portion 104A and a small-diameter portion 104B. A yoke 105 is fitted onto the large-diameter portion 104A at an end thereof and is coupled thereto by a pin. A wedge 107 is fitted onto the small-diameter portion 104B and is pushed by a coil spring 106 so as to come in contact with an end surface of the large-diameter portion 104A. The wedge shaft 104 is inserted in a hole 108 formed in the housing of the transmission, and a supporter 109 having a tilted surface is arranged in front of the wedge 107. A foot portion formed on the yoke 105 fits to an end of a parking lever 110 which is fixed to a lever shaft 110A so as to turn.
A speed-change lever operated by a driver is coupled to a selector lever 111A fixed to an end of a parking operation shaft 111. A plate 111B is fixed to the parking operation shaft 111. If the driver operates the speed-change lever in the A-direction up to a parking position, the parking lever 110 turns via the plate 111B, pin 111C and auxiliary lever 110B, and the wedge shaft 104 moves in the B-direction in the left drawing (toward the left in the right upper drawing). The wedge 107 fitted onto the wedge shaft 104 is pushed by the coil spring 106 (compression coil spring), moves together with the wedge shaft 104, and its lower surface slides on the supporter 109 while its upper surface pushes the parking pawl 103 up in the C-direction. Namely, the wedge 107 works as a cam for pushing the parking pawl 103 up, causing the pawl portion 103A of the parking pawl to be brought into mesh with a dented portion of the parking gear 102 to block the turn of the output shaft 101 of the transmission.
Upon operating the speed-change lever to the parking position, the parking pawl 103 is pushed up. At this moment, the pawl portion 103A of the parking pawl is not always at a position where it is in agreement with the dented portion of the parking gear 102 but often comes in contact with the protruded portion thereof. If the pawl portion 103A comes in contact with the protruded portion, motion of the wedge 107 is interrupted on its way. Therefore, the wedge shaft 104 only moves in the B-direction. However, since the wedge 104 has been pushed by the coil spring 106, the parking gear 102 fixed to the output shaft 101 slightly turns and the pawl portion 103A comes into agreement with the dented portion. Then, the wedge 107 moves toward the left permitting the pawl portion 103A of the parking pawl to come in mesh with the dented portion of the parking gear.
The parking lock device is a mechanism mounted in the last part of the transmission and causes an increase in the overall length of the transmission. A vehicle has only a limited space for arranging the power transmission device. Therefore, the parking lock device must be constituted as compactly as possible and it is, particularly, desired that the parking lock device has a reduced size in the axial direction of the transmission.
Further, the packing lock device is provided for the AT vehicles mounting an automatic transmission equipped with a torque converter but is not usually provided for the MT vehicles capable of so-called gear-engaged parking with the transmission being coupled to the engine through a friction clutch. However, there a variety kinds of vehicles used for various purposes. For example, a truck may be equipped with a crane for loading and unloading cargos, and the power for operating the crane may be taken out from the engine mounted on the vehicle by using a power take-out shaft called PTO shaft. When the above work vehicle equipped with the PTO shaft is in operation while parking the vehicle, the engine is running with the transmission at the neutral position. Therefore, the vehicle cannot be gear-engage parked. Should the vehicle starts moving during the working, the operation will be seriously hampered often causing unexpected damage to the surrounding. Though the vehicle is usually equipped with a parking brake, the vehicles having the PTO shaft may often require another braking system for reliably maintaining the vehicles in a halted state during the working.
When the MT vehicle mounting the transmission of the type of parallel shaft gear mechanism is to be additionally furnished with a parking lock device to reliably prevent the vehicle from unexpectedly moving, it is desired that the parking lock device is mounted without changing the shape and size of the existing transmission as much as possible. Further, if the basic shape and size of the transmission are maintained unchanged, the basic transmission can be used in common for manufacturing a variety of kinds of vehicles irrespective of whether the parking lock device is provided, making it possible to decrease the cost for the production or assembling.
The problem of the present invention is to mount a parking lock device on a transmission without modifying the constitution of the base transmission as much as possible, the parking lock device being constituted in a compact size and having a short length in the axial direction of the transmission.